Damage detection method and apparatus for machine elements utilizing vibrations therefrom

ABSTRACT

A system for detection of damage to a machine element in rolling engagement with another machine element utilizing vibrations produced by such damage. The peak value of the vibration signal obtained from the machine elements is compared with the mean value of the rectified vibration signal to obtain a ratio which is an indication of and the extent of the damage to the machine element.

United States Patent Weichbrodt et a].

[451 July 18,1972

[54] DAMAGE DETECTION METHOD AND APPARATUS FOR MACHINE ELEMENTSUTILIZING VIBRATIONS THEREFROM Bjorn Weichbrodt; Bernard Darrel, both ofSchenectady, N. Y.

General Electric Company Oct. 30, 1970 [72] Inventors:

Assignee:

Filed:

Appl. No.:

US. Cl ..73/67, 340/261 Int. Cl. Field of Search ..73/67, 71.4, 69;340/261 hmmwuu [56] References Cited UNITED STATES PATENTS 3,095,7307/1963 Matheson ..73/67 3,486,616 12/1969 Brany et al ..73/67 X PrimaryExaminer-Richard C. Queisser Assistant Examiner-Arthur E. KorkoszAttorney-Paul A. Frank, John F. Ahern, Julius J. Zaskalicky, Frank L.Neuhauser, Oscar B. Waddell and Joseph B. Forman [57] ABSTRACT A systemfor detection of damage to a machine element in rolling engagement withanother machine element utilizing vibrations produced by such damage.The peak value of the vibration signal obtained from the machineelements is compared with the mean value of the rectified vibrationsignal to obtain a ratio which is an indication of and the extent of thedamage to the machine element.

7 Chins, 5 Drawing Figures DAMAGE DETECTION METHOD AND APPARATUS FORMACHINE ELEMENTS UTILIZING VIBRATIONS THEREFROM This invention relatesto a method and apparatus for the early detection of faults in machineelements in rolling engagement such as a hearing or gear elements.

Techniques for the detection of incipient failure of bearings in currentuse are either visual and require costly disassembly and reassemblyprocedures, or depend upon the amplitude of the vibration signalsgenerated by the bearing itself. The amplitude of the vibration signalsfrom the bearing is not necessarily indicative of the condition of thebearing. New bearings from different manufacturers will producediiferent amplitudes of signal as a result of variations in finalgrinding techniques used thereon. Variations in amplitude of vibrationsignals from bearings made by the same manufacturer as well cannot bereliably correlated to the condition of the bearings. In addition, noisesignals from new bearings are usually higher than those from a good usedbearing as the many grinding scratches produced in the manufacture ofthe bearing increase the overall noise signal. Under use the bearingsurfaces are gradually burnished and consequently the noise signalobtained therefrom is considerably reduced. As the bearing fatigues,pits or spalls are produced that cause large amplitudes of signal toappear over only a small portion of a cycle of operation. At this timethe overall vibrational level can still be small.

Accordingly, techniques relying on the overall vibration signal levelfrom the hearing are not completely reliable or sufficiently sensitiveto provide an indication of incipient failure. In a copending patentapplication, Ser. No. 23252, filed Mar. 27, 1970, and assigned to theassignee of the present invention there is disclosed a technique for theseparation of peak signals from vibration or sonic signals from bearingor gear assemblies and correlating the peak signals with localizeddefects in elements of the bearing or gear assemblies for the purpose ofdetermining incipient failure. The present invention is directed toproviding a simplification in the technique described in theaforementioned application suitable for use in certain cases where therequirement for signal sensitivity is not quite as high.

Accordingly, an object of the present invention is to provide means forprocessing of vibration signals measured externally on an assembly ofengaging rolling elements to provide information about the existence ofdefects in the rolling elements.

Another object of the present invention is to provide a simple means forproviding an indication of the presence of peaks in a signalcorresponding to incipient bearing damage or failure regardless of theabsolute level of the vibration signal coming from the bearing.

Another object of the present invention is to provide a vibrationprocessing means for the detection of defects in rolling elements inengagement which can be easily automated.

Another object of the present invention is to provide simple apparatuscapable of detecting small surface defects in bearings while normallyoperating in noisy environments.

In carrying out the invention as applied to detection of damage in abearing assembly, vibrations of the bearing assembly are sensed andconverted into an electrical signal during the operation of thebearings. The electrical signal is filtered in a wide bandpass filter toenhance the significant signal components thereof. Means are providedfor deriving a second signal from the electrical signal corresponding tothe peak value thereof. A signal processing circuit is also provided forderiving a third signal from the electrical signal corresponding to amean value of the rectified electrical signal. A ratio circuit isprovided for deriving a ratio signal corresponding to the ratio of theamplitudes of the second and third signals. A level responsive indicatoris provided responsive to a predetermined level of the ratio signalcorresponding to a peak of a second predetermined level in theelectrical signal in relation to the mean value thereof.

The novel features which are believed to be characteristic of thepresent invention are set forth in the appended claims. The inventionitself together with further objects and advantages thereof may best beunderstood by reference to the following description taken in connectionwith the accompanying drawings wherein:

FIG. I is a sectional view of part of a bearing showing an inner racewith a defect, an outer race and one ball.

FIG. 2 is a block diagram of the apparatus of the invention partly inschematic form showing an embodiment of the invention as applied to thedetection of a defect in the bearing illustrated in FIG. 1.

FIG. 3 shows a graph of an electrical signal representing the amplifiedand filtered output of the vibration transducer of FIG. 2 in response tothe defect in the bearing shown therein.

FIG. 4 shows a diagram partly in schematic form of apparatusillustrating a modification of the apparatus of FIG. 2 applied to thedetection of defects in a gear transmission.

FIG. 5 shows a graph of a signal obtained at the output of the apparatusof FIG. 4 when monitoring a gear with a defect in the gear transmissionthereof.

A rolling element bearing, if properly designed and operated, performsits function without appreciable wear of the bearing surfaces. But evenif the wear is negligible the cyclical loads of the rolling elementspassing over the bearing surfaces cause fatigue of the material so thatafter a time surface defects begin to appear. These defects are at firstfew and local. The time before the initialdefects occur may be greatlyshortened by excessive temperature, lubrication failure, corrosion, etc.and can therefore normally not be calculated. Once the initial surfacedefects have developed, the bearing enters into a new phase of its life,characterized by considerably higher wear or destruction rate, andfinally fails to perform its function. To predict bearing failure, it istherefore essential to detect the first surface defects while they arefew and local.

FIG. 1 shows a part of a bearing 1 including an inner race 2 with adefect 3 located therein, an outer race 4 and a ball 5 included betweeninner and outer races. The present invention makes use of the fact thatan impact is generated every time a defect in an otherwise smoothsurface comes into rolling contact with any other smooth surface. Forexample, the dent 3 in the inner race 2 of the ball bearing 1 generatesan impact every time a ball rolls over it. As a result of thesymmetrical shape of a bearing the defect generates a sequence ofimpacts approximately equally spaced in time provided the bearingrotates at substantially constant speed. The time interval between twosubsequent impacts depends on the bearing speed, geometry and thelocation of the defect.

Reference is now made to FIG. 2 which shows in section the bearing 1with the inner race 2 supporting shaft 10 and the outer race 4 mountedon the support structure 11. The detection system in accordance with thepresent invention includes an accelerometer 15 which is mounted on thesupport structure 11 adjacent the bearing 1, a bandpass filter 16 whichallows a predetermined frequency range of the vibration signal from theaccelerometer to be passed therethrough, an automatic gain controlamplifier 17 for amplifying the signal from the bandpass filter. Alsoincluded are a peak detection circuit 18 including a time constantnetwork 19 for detecting peak values of signal, a root means squaredetection circuit 20 for obtaining the root mean square value of signalfrom the a.g.c. amplifier 17, a divider circuit 21 for the ratio of thepeak amplitude to the root mean square value of signal, a long timeconstant circuit 22 for minimizing the effect of singular transientnoise, a meter relay device 23 which is responsive to a predeterminedlevel of signal for actuation of a suitable indication means such as analarm 24. The accelerometer 15 converts mechanical vibrations into anelectrical signal in which the amplitude of the electrical signal variesin accordance with the acceleration component of the mechanicalvibrations. Such a device is particularly sensitive to impacts. Theaccelerometer is mounted on the support 11 close to the bearing 1 toproduce good response to vibrations from the bearing. The filter 16 is aconventional filter for eliminating background noise and for passing thepeak signals which it is desired to detect. The automatic gaincontrolled amplifier 17 is a conventional automatic gain controlledamplifier. The gain of the amplifier preferably is controlled by thepeaks of the electrical signal to maintain a predetermined level ofoutput and to avoid clipping of large peaks in the electrical signal. Ifdesired the gain of the amplifier may be controlled in response to amean value of the rectified electrical signal. The output of theamplifier 17 is supplied to the peak detection circuit 18 and to theroot means square detection circuit 20.

The peak detection circuit 18 includes an input terminal 30, and outputterminal 31 and a common input-output terminal 32. The peak detectioncircuit also includes unilaterally conducting device or rectifier 33having an anode 34 and a cathode 35. The anode 34 is connected to theinput terminal 33, the cathode 35 is connected to the output terminal 31and common terminal 32 is connected to ground. A capacitor 36 and aresistor 37 forming time constant network 19 are connected in parallelbetween the output terminals 31 and 32. Alternating voltage from theamplifier 17 is applied to the input terminals 30 and 32 of the peakdetection circuit 18 and is rectified and peak voltage appears acrossthe output terminals thereof. The peak voltage appearing across theterminals 31 and 32 decays in amplitude in accordance with the timeconstant of the network 19. The time constant of the network 19 isselected so as to substantially retain the voltage developed thereacrossduring the interval between peaks in the electrical signal produced bythe repeated impacts in the bearings.

The output from the amplifier 17 is also applied through a potentiometer38 to a circuit for determining a mean value of the electrical signal.In the embodiment shown the detector circuit 20 is shown as a root meansquare detector. The root mean square detector develops an output whichis the root mean square value of the alternating signal applied to theinput thereof and represents the average energy content of the signal.The root mean square detector may be a detector such as Series 742/9742made by Transmagnetics of Flushing, New York. Other circuits forderiving a mean value of a signal of course may be utilized. The outputof the root mean square detector is applied to the denominator terminal41 of a divider 21. The output from the peak detector 18 is appliedthrough a buffer amplifier 39 and a potentiometer 40 to the numeratorterminal 42 of the divider 21. From the quotient terminal 43 of thedivider 21, a signal is obtained which represents the quotient of thesignals applied to the numerator and denominator terminals andrepresents the ratio of the peak signal to the root mean square signalobtained from the accelerometer. The potentiometers 38 and 40 functionto vary the sensitivity of the apparatus through adjustment of the gainsof the signal channels feeding the numerator and denominator terminalsof the divider 21. The divider 21 may be any of a number of dividers,for example, such as analog divider series 450 made by Transmagnetics ofFlushing, New York.

The long time constant circuit 22 includes an input terminal 50, anoutput terminal 51 and a common input and output terminal 52. The longtime constant circuit 22 also includes a series current limitingresistor 53, a unilaterally conducting device or rectifier 54 having ananode 55 and a cathode 56, a time constant network including capacitors58 and 59 of different capacitances, resistor 60 and a switch 61including an arm 62 and three contacts 63, 64 and 65. The anode 55 ofthe rectifier is connected through resistor 53 to the input terminal 50,and the cathode 56 of the rectifier is connected to the output terminal51. The arm 62 of the switch is connected to the cathode 56 of therectifier. The capacitor 58 is connected between the contact 64 andterminal 52 and capacitor 59 is connected between the contact 65 andterminal 52. The resistor 60 is connected between the output terminals51 and 52. The time constant of the output circuit of the long timeconstant circuit 22 may be controlled by setting of the arm 62 of theswitch to parallel the appropriate capacitors 58 or 59, in shunt withthe resistor 60 to provide the desired time constant. The time constantof the output of the circuit 22 is selected so that it is relativelylarge in relation to the period of rotation of the bearing element underinvestigation. Accordingly, it has an averaging effect on the output ofthe divider 21 when the output signal from the accelerometer 15 includesa periodic peak signal while producing an output that varies somewhatfrom one revolution to the next resulting from stray and random causes.The output from the long time constant circuit 22 is applied to a meterrelay 23. The meter relay may be any of a variety of such devicescommonly available, for example, the noncontacting meter relay made bythe Instrument Department of General Electric Company located at Lynn,Massachusetts, which are responsive to a given level of input foractuation of a pair of contacts. Closure of the contacts of the relay isutilized to actuate an alarm 24 connected thereto to provide anindication that a particular level of an input has been applied to themeter relay 23. The knob 25 of the meter relay sets the pointer 26 onthe face of the meter relay to a desired level. When the input signalcauses a meter element of the relay to be aligned or exceed that level,the contacts of the relay are actuated without loading the input circuitof the meter relay to effect the operation of external apparatus such asthe alarm 23.

Reference is now made to FIG. 3, which shows a graph 74 of the signalproduced by the bearing 1 and sensed by accelerometer 15 of FIG. 2. Theordinate on the graph represents voltage amplitude and the abscissarepresents time. The impact peaks 75 in the signal are separated by atime period T representing the period of rotation of the inner race 2 ofthe bearing of FIG. 1 which has the defect 3 on it. The pointer 26 onthe meter relay is set for a predetermined value of input appliedthereto. For example, such value may correspond to a ratio of peak toroot mean square amplitude value represented by the level 76 of thegraph of FIG. 3. Accordingly, any peak exceeding this level wouldactuate the meter relay 23 which in turn would actuate the alarm 24which then provides a visual or aural indication of the fact that thelevel has been exceeded. In the absence of any defect in the bearingbeing monitored no regularly occurring peak signals would appear,although occasionally transient peaks from external sources would appearwhile such peaks are detected by the peak detector 18, they are rejectedby the long time constant circuit 22 and consequently do not actuate themeter relay 23. The time constant of the time constant network may be,for example, 100 times the period of rotation of the bearing elementunder consideration. In the operation of the system, should the level ofthe signal from the accelerometer change for various reasons, the ratioobtained from the divider would not change as both the root mean squarevalue and the peak amplitude would change at the same time. Accordingly,the apparatus may be set to provide an alarm when peak to a mean valueof the signal exceeds a predetermined value and such setting would nothave to be changed when the apparatus is used with different bearings.

The apparatus of FIG. 2 may also be utilized for the detection ofdefects in gear assemblies as well as in bearing assemblies. FIG. 4shows a gear transmission 80 including a plurality of gears 81, 82 and83 which are mounted on respective shafts 84, 85 and 86 and supported inbearings (not shown) in the housing of the transmission. The gear 81meshes with the gear 82, which in turn meshes with gear 83. Power may beapplied to the shaft 84 and taken from shaft 86. Gear 84 has the largestnumber of teeth and gear 83 has the smallest number of teeth. The tooth87 of gear 82 has a surface defect 88, for example, it is scored, andprovides an uneven meshing surface with the teeth of gear 84. Rotationof the gear 84 at constant speed produces a series of impacts equallyspaced in time. Gear assemblies generate considerable noise and defectsmay occur in gear elements other than the one being specificallymonitored, consequently a complex series of peaks may be produced. Ifthe gear assembly involves a few elements, a sensor may be placed closeto the gear element being monitored and the signal therefrom processeddirectly by the apparatus of FIG. 2 as shown for processing bearingsignals. However, in

' gear transmissions involving a large number of gears the vibrationsfrom the transmission sensed by a sensor need to be further processed orthe signal enhanced to obtain the signal which is representative orcharacteristic of the vibration produced by the particular gear elementbeing monitored. The manner in which such characteristic vibrationsignal is obtained is also shown in FIG. 4.

The apparatus of FIG. 4 also includes a preamplifier 91 which amplifiesthe signal from the accelerometer, a bandpass filter 92 which eliminatesextraneous background vibrations or noise signals, an amplifier 93 forthe filtered signal, an optional rectifier 94 and a summation analyzer95 to which the rectified signal is applied. The rectification of theamplified signal serves to avoid signal cancellation in summationanalyzers which otherwise might happen if consecutive signal elements tobe summed are not exactly identically positioned in time. Rectificationis not always needed for further analysis. The apparatus furtherincludes a tachometer 96 mechanically coupled to the drive shaft 84, afrequency converter and summation analyzer synchronizer 97. Synchronizer97 converts the signal from the tachometer to a trigger signal havingthe same periodicity as the periodicity of the gear 82 being monitored.The trigger signal is applied to the summation analyzer 95 tosynchronize the operation thereof.

The summation analyzer 95 is an apparatus such as Signal Analyzer type5480B made by Hewlett Packard Co. of Santa Clara, California, which sumsa plurality of signal sampling cycles, each cycle including a pluralityof samples, and which provides anaveraged output of one cycle of thesummed sampling cycles. The summation analyzer is provided with an inputterminal to which the signal to be analyzed is applied and asynchronizing terminal to which a synchronizing or trigger pulses fromsynchronizer 97 are applied to initiate the sampling cycles. Onceinitiated the summation analyzer takes a predetermined number of samplesin sequence and stores the signal level of each sample in its memory.When it is again triggered by a subsequent trigger pulse, the samplingcycle is repeated and each sample of the cycle is added to a respectivesample of the preceding cycle or cycles. After a predetermined number ofcycles have been executed and averaged, the summation analyzer isautomatically operated in the display or readout mode in which theaveraged sum of the cycles of signal samples are supplied to the outputterminal of the apparatus for display or further signal processing, asdesired. The number of signal samples taken per cycle can be set asdesired and the duration of the cycle of samples may also be set asdesired to match the cyclical phenomena under study.

The output from the summation analyzer 95 is applied to the input ofage. amplifier 71 of FIG. 2 in place of the signal from the filter 17.The signal from the output of a. g.c. amplifier 17 is processed in thesame way as for the detection of hearing defects by the peak detector18, the RMS detector 20, the divider 21, the long time constant circuit22, the meter relay 23, and alarm 24 to provide an indication of a geardefect.

FIG. 5 shows graph 98 of vibrations with peaks 99 obtained from thesummation analyzer 95 and produced by defect 88 in the gear 82. Theordinate on the graph represents voltage amplitude and the abscissarepresents time. A predetermined level of the peak 99 in relation to theroot mean square value of the total signal 98 is automatically indicatedby means of the apparatus shown in FIG. 2 by appropriate setting of themeter relay pointer 26 on its scale to correspond to the peak level 100.A particular advantage of the apparatus embodying the invention is thatit does not depend on absolute levels of the signal, but only onrelative measurements. Thus, the apparatus is not sensitive to changesin sensor or amplifier sensitivity, nor to the exact location of thesensor as long as the sensor is close to the housing of the bearing orgear.

While the invention has been described in specific embodiments it willbe appreciated that modifications may be made by those skilled in theart and we intend by the appended claims to cover all such modificationsas fall within the true spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

l. A system for detecting damage to a rolling element in a pair ofengaging rolling elements, said damage producing a component ofvibration having a periodicity corresponding to the periodicity ofrotation of said rolling element and having a peak above the level ofother vibrations of said rolling elements,

a vibration sensor coupled to said rolling element for sensing thevibrations thereof and converting said vibrations into a firstelectrical signal,

means for deriving a second signal from said first signal correspondingto the peak value of said first electrical signal,

means for deriving a third signal from said first signal correspondingto a mean value of the amplitude of said elec trical signal,

means for deriving a signal corresponding to the ratio of the amplitudesof said second signal and said third signal, and

indicator means responsive to a predetermined level of said third signalcorresponding to said peak of said component of vibration, whereby saiddamage is detected.

2. The combination of claim 1 in which signal enhancement means areprovided to enhance the amplitude of the component of vibration inrelation to the amplitude of the other vibrations.

3. The combination of claim 1 in which said rolling element is a rollingelement of a bearing.

4. The combination of claim 1 in which said pair of rolling elements aregears in which the teeth of one of said gears engage the teeth of theother of said gears.

5. The combination of claim 1 in which said means for deriving a thirdsignal derives a mean value signal corresponding to the average value ofthe rectification of said first electrical signal.

6. The combination of claim 1 in which said means for deriving a thirdsignal derives a mean value signal corresponding to the root mean squarevalue of said first electrical signal.

7. The combination of claim 1 in which a network is interfaced in thesignal path from said means deriving said third signal to said indicatormeans, the time constant of said network being large in relation to theperiod of rotation of said rolling element.

1. A system for detecting damage to a rolling element in a pair ofengaging rolling elements, said damage producing a component ofvibration having a periodicity corresponding to the periodicity ofrotation of said rolling element and having a peak above the level ofother vibrations of said rolling elements, a vibration sensor coupled tosaid rolling element for sensing the vibrations thereof and convertingsaid vibrations into a first electrical signal, means for deriving asecond signal from said first signal corresponding to the peak value ofsaid first electrical signal, means for deriving a third signal fromsaid first signal corresponding to a mean value of the amplitude of saidelectrical signal, means for deriving a signal corresponding to theratio of the amplitudes of said second signal and said third signal, andindicator means responsive to a predetermined level of said third signalcorresponding to said peak of said component of vibration, whereby saiddamage is detected.
 2. The combination of claim 1 in which signalenhancement means are provided to enhAnce the amplitude of the componentof vibration in relation to the amplitude of the other vibrations. 3.The combination of claim 1 in which said rolling element is a rollingelement of a bearing.
 4. The combination of claim 1 in which said pairof rolling elements are gears in which the teeth of one of said gearsengage the teeth of the other of said gears.
 5. The combination of claim1 in which said means for deriving a third signal derives a mean valuesignal corresponding to the average value of the rectification of saidfirst electrical signal.
 6. The combination of claim 1 in which saidmeans for deriving a third signal derives a mean value signalcorresponding to the root mean square value of said first electricalsignal.
 7. The combination of claim 1 in which a network is interfacedin the signal path from said means deriving said third signal to saidindicator means, the time constant of said network being large inrelation to the period of rotation of said rolling element.